This Week In The Lab: a nicely engineered underwater camera and lighting system. All beautifully turned, milled and TIG welded stainless steel, this thing can go deep and withstand some rough treatment. It was seriously heavy!

The exact installation environment wasn’t known. Since it was expected to be operated in harsh conditions we opted to test to the generic industrial standards EN 61000-6-2 for immunity and EN 61000-6-4 for emissions.

A Simple EMC Fix

Just one fix required: under 10V/m radiated RF immunity testing one of the positioning motors wasn’t responding to it’s control signals. The control from user to motor was all digital so interference on those lines was unlikely.

The fault finding process was relatively straightforward this time.

We quickly figured out that the problem lay with the optical sensor that detected the shaft position and set the end stops for the range of motion. It was being triggered by the noise which caused it to think that the shaft was simultaneously at both of its end positions.

A ferrite core around the cable and a decoupling/filtering capacitor on the sensor input to the controller stopped the noise from affecting performance.

RF current transformers (or probes) are commercially available products from places like Fischer CC or Solar Electronics and they work really well, have specified bandwidth and power handling characteristics, built in shielding, robust case, etc.

They also cost a few hundred £$€ each which, if you are on a budget like most people, represents a significant investment for a individual or small laboratory. However, this one can be built very cheaply; most labs will have a development kit with some clip on ferrite cores, if not the core I used only costs £5 from RS.

DIY Current Probe

I’m a big fan of making my own test adaptors and equipment as its a great way to really understand how things work and the compromises in any design. As such I decided to share how I go about making this kind of really useful tool.

It’s primary use is for A-B comparison work; measuring the current, performing a modification and then measuring the current to see the improvement.

It is to be stressed that my version is a crude but effective piece of equipment and does not replace a well designed commercial product. There’s a time and a place to invest in quality equipment and one should use engineering judgement on when that is. For instance, measuring the RF current accurately is definitely a job for a properly designed and characterised device.

If you want to explore RF current transformers in more detail then there is plenty of info on Google, but these links are useful places to start.

Some of the design compromises involved in this low cost approach include:

Core Losses / Insertion Loss

The ferrite material in these cores is specifically designed to be lossy at the frequencies of interest, which will result in a lower reading than a higher bandwidth core and a reduction in the amount of noise on the cable downstream from the noise source. This can in some cases mask the effect you are trying to measure. The commercially available products use low loss, high bandwidth ferrite cores.

A high insertion loss also makes these parts more unsuitable for injecting noise into circuits for immunity testing. they can be calibrated for this task using a simple test setup (to be covered later)

Secondary Turns

Number of secondary turns controls sensitivity but the more you add, the inter-winding capacitance increases, decreasing the bandwidth of the tool. I generally use 5 or 6 turns to start with but I do have a 20 turn part made with micro coax on a solid core which also helps to deal with…

Capacitive pickup

From the cable under test to the secondary winding. Normally a split shield (so that it doesn’t appear as a shorted turn) is built in to commercial products. Guess what, that’s easy to do on this with a spot of copper tape or foil.

Not as Robust

Although a well designed product, the plastic hinges and clips on the cores are not designed for repeated opening and closing. The Wurth Elektronik system of a special key to open and close the core is much more robust at the expense of having to keep a few keys to hand for when they inevitably go missing. However these parts are so cheap and quick to make that a broken clip on core is no real obstacle.

Future Videos

I’ll be following this video with some hints and tips on how to use these devices effectively for finding radiated emissions problems and for looking at conducted RF immunity issues. Stay tuned.

Video and Construction Errata

The sharp eyed of you will have spotted that I originally assembled the BNC connector on the core so that it covered the key-way to open the clamp. I rectified this but didn’t film the change.

Also, you can wrap the wire round the core without removing it from the housing but that means you don’t have a nice flat surface to affix the BNC connector to. It does make it easier to close the clamp however so make your choice.

These little converters are super handy to interface between your modern PC or laptop and the simpler, lower technology RS-232 serial port used by many pieces of equipment for control or debug purposes. However, like any commodity item there are design compromises, including EMC ones, that you need to be aware of.

I was recently performing some Electrical Fast Transient (EFT) testing on a customers product and was surprised to observe it failing at quite a low level of injected transient of 200V. It appeared that the whole system crashed when the bursts were applied to any of the digital I/O ports.

Even more confusing was that I’d looked over the schematic and the port protection measures that they had implemented were very sensible with ferrite beads and diode clamps.

A pointer came from observing the front panel of the device with all of it’s indicator LEDs blinking away as if it was working properly. Yet the equipment under test (EUT) wasn’t responding to serial communications and the TeraTerm serial port software was still showing a connection.

Checking through the test setup, I theorised that the RS-232 to USB converter that I was using might be crashing or responding to the EFT pulse as a start bit to a frame. Despite being isolated with a Coupling/Decoupling Network (CDN), when a scope probe was added to the RXD line on the decoupled side of the CDN, a transient with 30V of pk-pk amplitude was visible when the EFT burst was applied.

I tried two other converters that I had in the lab and none of them were happy with this pulse and also refused to work correctly.

So I knocked up a small filter PCB with a pi filter on each line (RXD, TXD and 0V) consisting of 2 x 100pF capacitors and a ferrite bead. The non-line side of the caps was taken to the HF ground plane using some adhesive copper tape (the EMC scoundrel’s last resort!) to return the currents back to the generator and not into the converter.

Success! No more interference and the converter works perfectly.

As an experiment (OK, I got slightly distracted by something interesting) I played around optimising the filter and managed to get it down to just two components – a 100pF capacitor on the TXD and RXD lines of the converter.

Now I know that these devices will be designed to the lowest price point but two 0402 capacitors is hardly breaking the bank! It does make you wonder how they managed to get through their own EMC testing, if at all.

Incidentally when this was later tested in the chamber it had some fairly strong 12MHz harmonics from the USB 1.2 data lines that only just squeaked under the limit line lending further weight to my suspicions of corner cutting and poor design!

So today’s lessons are:

Beware of cheap generic test adaptors and EMC issues caused by them – both immunity and radiated.

Consider your port filtering carefully. Many I/O interfaces can stand a small capacitor or filter adding to it and the benefits for EMC are significant. It gives a path for interfering signals to the local ground and will also improve your emissions too. The customer who’s product I was testing had such parts fitted; it passed the testing at 1kV EFT without issue (the spec is 0.5kV).

Using a fibre optic serial port adaptor would probably have helped here by increasing the common mode impedance of the connection (assuming of course it had been designed properly!)

A customer requested some support with one of their products, an IoT bridge device that takes various sensors and provides telemetry back to a central server using a GSM module. Some of the radio pre-compliance spurious emissions testing had suggested there might be some issues at certain frequencies.

After a couple of hours of radiated emissions measurements in the anechoic chamber and some bench work with some near field probes, I’d developed a pretty good idea of what was going on in terms of where the emissions were coming from and what their radiating mechanisms were.

Interestingly, there was a common theme to all of these emissions…

These features are common to a wide range of similar devices so some notes and a simple drawing (oddly I find sketching like this a good way to relax!) are presented in the hope it will give you some ideas about where your radiated emissions might be coming from.

The sketch shows a keypad board, a CPU board and a battery pack. Some other information is missing to permit a simpler drawing. All of these boards below sandwich together nicely into a plastic case which was the starting point for the investigation.

The problem frequencies identified were a 300MHz narrowband spike and a 250MHz broadband hump. Usually when I see broadband I think “power supply noise” and narrowband I think “digital noise”.

Let’s take a wander around the device.

Capacitive plate near field probing around (A) showed higher than background levels of 300MHz noise around the front panel button board. Since this was a “dumb” board, the noise was probably coming from the main CPU board. The noise emanating from the cable (B) was not appreciably higher but when approaching the CPU/memory the noise increased, the clock line between the memory device and CPU being the highest.

Two possibilities were that there was crosstalk on the PCB at (C) or perhaps inside the CPU itself but without getting into more complex analysis the exact cause is not known. Apart from the power lines, there was no extra HF filtering on the data lines, just a series resistor on the I/O lines of the CPU. The addition of a small capacitor (e.g. 47pF, either 0402 or an array) on each line to circuit ground forms an RC filter to roll off any unwanted HF emissions like this. I generally advocate making provision for such devices on the PCB but not fitting them unless required – better to provision for and not need than to require a PCB re-spin later in the development cycle.

Moving the near field probe around the bottom of the case where the battery lives (D) showed the broad 250MHz hump present on the battery. Unplugging the battery pack made the emissions drop by 10dBuV/m and measuring with a high bandwidth passive probe showed broadband noise present on the outputs of the battery charger (E) from the switching converter. Some low-ohm ferrite beads in series with the battery terminals will help keep this noise on board and prevent common mode emissions from the battery and cables (F).

Lastly, the antenna was unplugged and some other broadband noise was found on the cable (G) at 360MHz, this time from the main 5V DC/DC converter on the main PCB.

Conclusion

So what is the common theme? All the radiation problems stem from cables connected to the main PCB. As soon as you add a cable to a system you are creating a conductor with a poorly controlled return path or “antenna” as they are sometimes known in the EMC department!

Treat any cable or connector leaving your PCB as an EMC hazard. You have less control over the HF return paths in the cable environment than you do on the PCB. Apply appropriate HF filtering to the lines on the cable and remember that even a shielded cable can cause problems.

Sometimes, like the antenna cable, there’s not a lot you can do about it other than practice good design partitioning to keep noisy sources away from the cable and to apply a ferrite core around the cable if it becomes a problem during testing.

I hope you found this useful and that it has given you some pointers for looking at your own designs with a new perspective.

I’m really happy to have one of my blog articles featured on Interference Technology.

Problem solving and fault finding EMC problems, especially radiated emissions, is one of my specialities and oddly enough is one of the facets of my job that I enjoy the most. After a successful exercise in helping a customer out with their product, getting the chance to write about it and share it with you is a real bonus.

Fixing radiated emissions is at it’s most challenging when the scope for modification to the unit are limited by the fact there are significant stock of PCBs or components that would require scrapping and redesign. Finding a way to use the existing stock was key in this example as the customer had significant time and money invested into the project. Thankfully I was able to help them out.

Head on over to Interference Technology and have a read through – I even put pictures in! Hopefully it will give you an idea of how I work and the sort of EMC issues that I can help you solve.

Many of the customers I deal with are technically savvy and extremely good at designing innovative and clever devices. I’m always learning something new every time I get a different product through the door. Unfortunately it isn’t practical or possible to be good at everything and EMC expertise, especially when it comes to fault finding and problem solving, can be hard to come by. This is where I come in.

I’ve been helping a good customer on a product that they’ve been working with that had some EMC troubles on a prototype design. It had originally been taken to a different test lab where they had performed a mains conducted emissions measurement showing a clear failure at low frequencies. There were a couple of other hard copy scans supplied where a capacitor value had been adjusted to try and improve the emissions but with no effect.

Mains Conducted Emissions Testing

I received the product and quickly set it up in our screened room to perform some EN 55014-1 conducted emissions measurements. Below you can see the first scan result, showing a failure of up to 10dB on the Quasi Peak detector. There’s clearly some room for improvement so let’s analyse the problem and see what we can do.

Our starting point for the improvement work

Lower frequency mains conducted emissions are not uncommon and are usually caused by differential mode voltage noise. This is generated by current flowing through the impedance presented by the primary side bulk decoupling and switching circuit. The switching frequencies of the power supply controller are usually in the 30 kHz to 250 kHz range putting it (and it’s harmonics) right in this lower frequency (sub 1MHz) range for this test.

Improving differential mode noise can be done in a number of ways. Removing the noise at source is the approach I advocate, in this case this can be achieved by reducing the impedance of the rectified mains bulk decoupling capacitor. A review of the BOM showed that the units had been built with some general purpose electrolytic capacitors with a relatively high impedance. So the first thing that I did was to swap out these parts for ones from the Nichicon PW series of low impedance electrolytic capacitors.

Changing the electrolytics to a low impedance variety

The result: a big improvement on the QP measurements, bringing some of them down by around 10dB. The improvement on the Average detector readings was less pronounced, especially around 550 kHz where only a 3dB improvement was registered. It is likely that the HF impedance of the decoupling capacitor is still a problem. One option is to apply a suitably rated high frequency decoupling capacitor in parallel with the bulk decoupling capacitor. The other option is to improve the filtering on the AC mains input to prevent the noise from escaping back down the line.

Filtering for differential mode noise can be provided in several ways. The most common method is to make an LC filter from the leakage inductance of a common mode choke paired with a Class X safety capacitor between Live and Neutral. The leakage inductance is in the tens of micro-Henries whereas the common mode inductance is often a couple of magnitudes larger up in the tens of milli-Henries. Simplistically (there are other effects to consider) a 10uH leakage inductance paired with a 470nF capacitor will roll off frequencies above 100 kHz. Well, let’s try that!

Now with an additional 470nF Class X capacitor soldered across the mains input terminals

Performance is improved by around 5dB across a wide range of frequencies; indeed the improvement can be seen up to 15 MHz. This leaves a margin of around 2dB to the average limit line which is perhaps a bit close for comfort and I would generally recommend looking at a little more filtering to bring this down a bit further to allow for variations in production and tolerance of components. Options for further improvements could include a second Class X capacitor to form a pi filter but because of the low impedance of the differential mode noise this approach might not be as effective. Adding some inductance to form an LC filter with the bulk decoupling capacitor is another approach.

However this proved the case to the customer for a PCB redesign to make space for the larger bulk decoupling capacitors and at least one Class X capacitor.

Surge and Safety

Following on from this work, at the customers request, I carried out a full suite of EMC tests on the product to EN 55014-1 (emissions) and 55014-2 (immunity). One thing that I noticed was the sound of an electrical breakdown during the application of a differential mode surge test. Taking off the outer casing, I managed to catch the below arc on camera during a 1kV surge event.

Snap, crackle and pop.

The arc appeared around the resistor; desoldering and removing it from the PCB showed a couple of points where there was arcing between the resistor body and the trace running underneath it.

Arcing evidence on the PCB

This problem has occurred because the resistor R1 is in series with the Live phase and the trace underneath is connected to the Neutral phase. When mounted flush to the PCB normally, the resistor has only its outer insulation between live and neutral. Reviewing the relevant electrical safety standard for the product requires a minimum clearance (air gap) for basic and functional insulation is 1.5mm. This can be achieved by standing the resistor up on spacers to keep it away from the PCB but then it starts to approach VDR1 and Q4 meaning a considered manufacturing approach is required. This was another incentive for redesigning the PCB.

The take-away lesson from this finding is to consider the Z axis / third dimension when reviewing a PCB as it can be easy to see things purely in two dimensions!

I hope you found this case study useful and that it has given you some tools with which you can improve your designs.

If you need some EMC fault finding expertise then get in touch: I’d be happy to help and I love a good challenge!

It has been an good couple of weeks here at Unit 3 Compliance with some very interesting products to work on.

Firstly, I received confirmation from a previous customer that their product has passed the required EMC certification testing at their accredited laboratory with the modifications that we incorporated during our problem resolution work. This is excellent news for all concerned!

Then things started off with some radiated emissions EMC testing and fault finding on quite a complex and clever piece of medical laboratory equipment with multiple interconnected boards, display, motors and servos. A few problems were identified and feedback given to the customer about potential improvements.

Noisy DC motors were again the theme in some more radiated emissions testing, requiring additional suppression on the motor terminals, made all the more challenging by the tight mechanical constraints of the product. Differential mode suppression on the terminals using ferrite beads to reduce the brush noise is the most effective solution but without a well defined RF return path to the brushes any noise reduction will ultimately be a compromise. More testing is being performed with some small filter PCBs mounted right on the motor terminals.

Lastly, our screened room test facility is almost completed and is being used for some mains and DC port conducted emissions testing with buck converter switching noise providing a challenge.

I’ve been working with a customer whose product had significant broadband radiated emissions throughout the spectrum, helping them with fault finding and improving their EMC performance. Partial information from their certifying test lab appeared to show a regular harmonic series suggesting noise from a digital clock of some kind. However, near field probing with a spectrum analyser on the bench disproved this theory and identified the brushed DC motor as the cause. It is believed that a combination of the certifying test lab only performing one frequency sweep, the long cycle time of the EUT and the random arcing noise from the DC motor brushes caused a series of regular peaks to appear distributed throughout the spectrum.

In reality, both spectrum analyser measurements and the radiated emissions plot shown below show a wide range of broadband noise once the graph had multiple sweeps through the spectrum to enable it to become fully populated.

Working closely with the customer, we selected an alternate motor from the same manufacturer part with suppression capacitors built in to decouple the brush noise at the source; always the most efficient way of solving an EMC problem is to knock it on the head where is is being generated. Putting the EUT in the anechoic chamber showed that this provided a significant improvement, especially at higher frequencies but there remained an hump at around 150MHz to 300MHz.

We followed up with testing various ferrite cores around the DC motor power leads to further reduce the noise being conducted down the lead, leading to the selection of a small ferrite core with two turns around the motor DC supply leads.

Below can be seen the scan data showing before any modifications (blue) and after changing the motor and adding the ferrite core (green). Testing also revealed what appeared to be a satisfied customer 😉